This invention relates generally to an apparatus and method for making draw tape thermoplastic bags and as PCT EP86/00187 on Mar. 27, 1986, published as WO86/05875 on Oct. 9,1986 particularly to an apparatus and method for making draw tape plastic bags wherein the draw tape is sealed or welded to the layers of thermal plastic of the hem of the bag and wherein the sealing process is performed as a continuous. as opposed to an intermittent process.
Plastic bags characterized by a draw tape which, when pulled. causes the mouth to constrict to substantially closed condition are generally referred to as draw tape plastic bags. Plastic bags of the type described are typically comprised of a bag body folded at the bottom to form front and rear sheets which are ordinarily sealed at the side edges. The upper edges of the front and rear sheets. located opposite the bottom fold, define the mouth of the bag. An upper margin at the bag mouth is typically folded inside the bag to form a hem which is ordinarily sealed to the bag body to form a channel for accommodating the draw tape. Pulling excessible portions of the draw tape typically causes the bag mouth to constrict and close as previously described.
Draw tape bags of the type described are often manufactured by automatic bag making machinery at a relatively rapid rate. This is typically accomplished by passing a web of plastic material through a number of folding sealing, and operating stations Typically the web is formed of a linear low density polyethylene material that originates as a tube of film.
As such. the above-described hem is typically made and sealed automatically by a machine at a hem sealing station. Typically the hem is formed by first slitting the tube of low density polyethylene material to form a web of material in C-folded condition. As used herein. the term C-folded condition refers to a length of material which has been folded over unto itself so as to form what may be subsequently identified as a front sheet and a rear sheet which are joined by the fold at the bottom, and are unjoined at the top so as to approximate a letter "C" in cross section. After the web has been cut into C-folded condition, the hem is usually formed at a station wherein the top edges of the C-folded web are folded towards the existing bottom fold in the C-folded web on the interior of the C-folded section, to form a hem at the mouth of the bag. After the hem is formed a tape typically made of a high density polyethylene thermoplastic material, is inserted into the hem. The hem is then sealed leaving the draw tape slidably disposed in the hem.
To improve the performance of the draw tape closure the draw tape is preferably sealed at the hem near each of the side edges of the bag. In this manner. when the draw tape is pulled out; the draw tape pulls the portion of the bag where it is sealed or welded to the hem toward the center of the bag, thereby closing the mouth of the bag. Heretofore the seal or weld of the draw tape to the bag has been ordinarily accomplished by briefly stopping the movement of the web so that a reciprocating jaw type heat sealer forms the seal through all of the layers of the hem and draw tape. This intermittent motion has been found necessary since the seal is typically formed through seven total layers of polyethylene material That is, the seal must be formed through four layers of low density polyethylene material corresponding to the hem (two layers for each side of the bag) and through two layers of high density polyethylene material corresponding to the draw tape (one layer for each side of the bag).
Though the intermittently formed seal or weld of the draw tape to the hem provided by such an operation has been acceptable, the time required to stop the movement of the web while the reciprocating jaw type heat sealer is aligned with the section where the weld is to be formed however brief it may be, is undesirable. Indeed almost any stoppage of the web as it proceeds through the manufacturing process may have concomitant drawbacks in efficiency speed, and economy. This intermittent sealing arrangement typically produces about 30 bags/min. It is desirable to increase this production rate, thereby increasing manufacturing economy and efficiency.
Therefore, in view of the above it is a primary object of the present invention to provide an apparatus and method for continuously forming a seal or weld between the draw tape and a predetermined section of the hem with continuous, non-intermittent type motion.
It is a further object of the present invention to provide a rotary type apparatus and method for forming a seal between the draw tape and a predetermined section of the hem.
It is still a further object of the present invention to provide an apparatus and method for improving the efficiency of sealing a draw tape to a predetermined section of the hem while still providing a secure seal through all of the layers of the hem and draw tape.